1. Field of the Invention
The present invention relates to an ink jet head and a method of manufacturing thereof, and more particularly to an ink jet head for discharging ink droplets outwards from the interior of a vessel by applying pressure to the ink liquid in the vessel, and a method of manufacturing thereof.
2. Description of the Background Art
An ink jet method of recording by discharging and spraying out a recording liquid is known. This method offers various advantages such as high speed printing with low noise, reduction of the device in size, and facilitation of color recording. Such an ink jet recording method carries out recording using an ink jet record head according to various droplet discharging systems. For example, droplet discharge means includes an ink jet head utilizing pressure by displacement of a piezoelectric element, and a bubble type ink jet head.
Layered type and bimorph type ink jet heads are known as droplet discharging means utilizing a piezoelectric element. A layered type ink jet head and a bimorph type ink jet head will be described hereinafter with reference to the drawings as conventional first and second ink jet heads.
FIG. 52 schematically shows a sectional view of the structure of a first conventional ink jet head. Referring to FIG. 52, a first conventional ink jet head 310 utilizes layered type piezoelectric elements as the droplet discharging means. Ink jet head 310 includes a vessel 305 and a layered type piezoelectric element 304.
Vessel 305 includes a cavity 305a, a nozzle orifice 305b, and an ink feed inlet 305c. Cavity 305a in vessel 305 can be filled with ink 80. Ink 80 can be supplied via ink feed inlet 305c. Nozzle orifice 305b is provided at the wall of vessel 305. Cavity 305a communicates with the outside world of vessel 305 via nozzle orifice 305b. A layered type piezoelectric element 304 is provided in cavity 305a. Layered type piezoelectric element 304 includes a plurality of piezoelectric elements 301 and a pair of electrodes 303. The plurality of piezoelectric elements 301 are layered. The pair of electrodes 303 are arranged alternately to be sandwiched between respective piezoelectric elements 301, whereby voltage can be applied effectively to each piezoelectric element 301. A power source 307 is connected to the pair of electrodes 303 to switch the application of voltage by turning ON/OFF a switch.
According to an operation of ink jet head 301, the switch is turned on, whereby voltage is applied to the pair of electrodes 303. As a result, voltage is applied to each of the plurality of piezoelectric elements, whereby each piezoelectric element 301 extends in a longitudinal direction (the direction of arrow A.sub.1). Ink jet head 310 of FIG. 53 shows the state where each piezoelectric element 301 extends in the longitudinal direction.
The expansion of each piezoelectric element 301 in the longitudinal direction (in the direction of arrow A.sub.1) causes pressure to be applied to ink 80 in cavity 305a. Pressure is applied to ink 80 in the direction of arrows A.sub.2 and A.sub.3, for example. By the pressure in the direction of arrow A.sub.2 particularly, ink 80 is discharged outwards via nozzle orifice 305b to form an ink droplet 80a. Printing is carried out by a discharged or sprayed out ink droplet 80a.
FIG. 54 is a sectional view schematically showing a structure of a second conventional ink jet head. Referring to FIG. 54, a second conventional ink jet head 330 includes a vessel 325 and a bimorph 324.
Vessel 325 includes cavity 325a, a nozzle orifice 325, and an ink feed inlet 325c. Cavity 325a can be filled with ink 80 via ink feed inlet 325c. Nozzle orifice 325b is provided at the sidewall of vessel 325. Cavity 325a communicates with the outside world of vessel 325 via nozzle orifice 325b. Bimorph 324 is arranged within cavity 325a.
Here a bimorph is referred to a structure where two electrodes are cemented to either side of a plate of a piezoelectric element. Therefore, bimorph 324 includes a piezoelectric element 321 and a pair of electrodes 323. Bimorph 324 has one end attached and fixed to the inner wall of vessel 325. Nozzle orifice 325b is located at a position facing the free end of bimorph 324. A power source 327 is connected to the pair of electrodes 323 to control the application of voltage by turning on/off a switch.
According to an operation of a second conventional ink jet head 330, cavity 325a is filled with ink 80. Voltage is applied to the pair of electrodes 323. More specifically, piezoelectric element 321 is displaced by application of voltage, whereby the free end of bimorph 324 is displaced in the direction of arrow B.sub.1, i.e. is warped. Here, the switch is turned off to cease application of voltage to the pair of electrodes 323. This causes the free end of bimorph 324 to be displaced in the direction of arrow B.sub.2 to result in the state shown in FIG. 55.
Referring to FIG. 55, pressure is applied to ink 80 in the direction of, for example, arrow B.sub.3 as a result of displacement of bimorph 324. By this pressure in the direction of arrow B.sub.3, ink 80 is discharged from nozzle orifice 325b to form an ink droplet 80a. Printing is carried out by ink droplets 80a discharged or sprayed out from nozzle orifice 325b.
A bubble type ink jet head will be described hereinafter as a third conventional ink jet head.
FIG. 56 is an exploded perspective view schematically showing a structure of a third conventional ink jet head. Referring to FIG. 56, a third conventional ink jet head 410 includes a heater unit 404 and a nozzle unit 405.
Heater unit 404 includes a heater 401, an electrode 403, and a substrate 411. Electrode 403 and heater 401 connected thereto are formed on the surface of substrate 411.
Nozzle unit 405 includes a nozzle 405a, a nozzle orifice 405b, and ink feed inlet 405c. A plurality of nozzles 405a are provided corresponding to heater 401. Nozzle orifice 405b is provided corresponding to each nozzle 405a. Ink feed inlet 405c is provided to supply ink to each nozzle 405a.
The operating mechanism of the bubble type ink jet head of the above-described structure will be described hereinafter.
FIGS. 57A-57E are sectional views of a nozzle showing the sequential steps of droplet formation of the bubble type ink jet head.
Referring to FIG. 57A, current flows to heater 401 by conduction of an electrode (not shown). As a result, heater 401 is heated rapidly, whereby core bubbles 81a are generated at the surface of heater 401.
Referring to FIG. 57B, ink 80 reaches the heating limit before the preexisting foam core is activated since heater 401 is rapidly heated. Therefore, core bubbles 81a on the surface of heater 401 are combined to form a film bubble 81b.
Referring to FIG. 57C, heater 401 is further heated, whereby film bubble 81b exhibits adiabatic expansion. Ink 80 receives pressure by the increase of volume of the growing film bubble 81b. This pressure causes ink 80 to be pressed outwards of orifice 405b. The heating of heater 401 is suppressed when film bubble 81b attains the maximum volume.
Referring to FIG. 57D, film bubble 81b is derived of heat by the ambient ink 80 since heating of heater 401 is suppressed. As a result, the volume of film bubble 81b is reduced, whereby ink 80 is sucked up within nozzle 405a. By this suction of ink 80, an ink droplet is formed from ink 80a discharged outside orifice 405b.
Referring to FIG. 57E, further reduction or elimination of the volume of film bubble 81b results in the formation of an ink droplet 80a.
According to an operation of a third conventional ink jet head 410, printing is carried out by discharging or spraying out ink droplet 80a formed by the above-described process.
The first, second and third conventional ink jet heads 310, 330, and 410, respectively, of the above-described structure include problems set forth in the following.
First and second conventional ink jet heads 310 and 330 using piezoelectric elements cannot obtain a great discharging force while maintaining the dimension of ink jet heads 310 and 330 at its small level. This will be described in detail hereinafter.
In the case where a piezoelectric element is used, an ink droplet is discharged by the deformation of the piezoelectric element caused by applying voltage. A greater level of voltage must be applied to the piezoelectric element in order to increase the amount of deformation of the piezoelectric element. However, there is a limit in the increase of the voltage applied to the piezoelectric element in view of the breakdown voltage of the ink jet head. Under such a condition where the applied voltage value is restricted, a great amount of deformation of the piezoelectric element cannot be ensured.
In the first conventional ink jet head 310 shown in FIGS. 52 and 53, piezoelectric elements 301 are layered in the longitudinal direction to obtain a greater amount of displacement. More specifically, in ink jet head 310, voltage is applied in the unit of each of the layered piezoelectric elements 301 to obtain an amount of displacement from each piezoelectric element 301 effectively, resulting in a .relatively great amount of displacement in the longitudinal direction. However, this amount of displacement is not sufficient by the layered piezoelectric elements 301 due to the limited applied voltage.
When a PZT that can convert voltage into an amount of displacement most efficiently at the current available standard is layered as the piezoelectric element in the first conventional ink jet head 301 with a cross sectional configuration of 2 mm.times.3 mm and a length of 9 mm, the layered piezoelectric elements can be displaced only 6.7 .mu.m in the direction of arrow A.sub.1 at an applied voltage of 100 V.
An approach structure can be considered of increasing the number of layers of piezoelectric elements 301 in order to obtain a greater amount of displacement in ink jet head 310. However, increase in the number of layers of piezoelectric elements 301 will result in a greater dimension in the longitudinal direction of the entire layered piezoelectric element 304. This entire increase in the size of the layered piezoelectric element will lead to increase in the size of pressure chamber 305a in which the piezoelectric elements are arranged. Therefore, increase in the size of ink jet head 301 cannot be avoided.
Similar to the second conventional ink jet head 330 shown in FIGS. 54 and 55, displacement in the direction of thickness of bimorph 324 (the direction of arrow B.sub.1) cannot be increased since a great amount of displacement of the piezoelectric element per se cannot be ensured.
When a PZT is used as the piezoelectric element and the bimorph has a dimension of 6 mm in length, 0.15 mm in thickness, and 3 mm in width in the second conventional ink jet head 330, bimorph 324 is displaced only 12 .mu.m in the direction of arrow B.sub.1 with an applied voltage of 50 V.
An approach can be considered of increasing the entire length of bimorph 324 to increase the amount of displacement in the thickness direction. Although the amount of displacement (C.sub.1) in the thickness direction is relatively low in bimorph 324 having a short length as shown in FIG. 58, the amount of displacement (C.sub.2) can be increased if the entire length is lengthened. It is to be noted that FIG. 58 is a side view of the bimorph for describing the amount of displacement in the thickness direction of the bimorph.
However, increase in the entire length of bimorph 324 in order to obtain a greater amount of displacement leads to cavity 325a of a greater volume in vessel 325. Therefore, increase in the size of ink jet head 330 cannot be avoided.
Thus, there was a problem that formation of a multinozzle head in which nozzles are integrated becomes difficult if the dimension of first and second conventional ink jet heads 310 and 330, respectively, is increased.
First conventional ink jet head 310 and second conventional ink jet head 330 use a PZT as the piezoelectric element. This PZT can be formed by a thin film formation method (for example, sputtering). However, a PZT used in first and second ink jet heads 310 and 330 is increased in the film thickness of the piezoelectric element per se. It is difficult to form such film thickness at one time by a general thin film formation method. In order to form a thick piezoelectric element by a thin film formation method, the piezoelectric elements must be layered according to a plurality of steps. Such a manufacturing method is complicated and will increase the cost.
There is also a problem that the lifetime of a bubble type ink jet head is reduced in the third conventional ink jet head 410. This will be described in detail hereinafter.
According to the bubble type ink jet head 410 shown in FIG. 56, a film boiling phenomenon must be established to obtain a thorough bubble 81b on the basis of the process shown in FIGS. 57A-57C. It is therefore necessary to rapidly heat heater 401. More specifically, heater 401 is heated to approximately 1000.degree. C. in order to heat ink 80 to a temperature of approximately 300.degree. C. High speed printing is realized by repeating heating and cooling in a short time by heater 401. This repeated procedure of heating to a high temperature and then cooling will result in thermal fatigue of heater 401 even if a material such as H.sub.4 B.sub.4 superior in heat resistance is used for heater 401. Thus, bubble type ink jet head 410 has the problem of deterioration of heater 401 to result in reduction in the lifetime of the ink jet head.